The present invention relates to a ferroelectric thin film coated substrate to be used in a ferroelectric memory device, a pyroelectric sensor device, a piezoelectric device, etc., a producing method thereof, and a capacitor structure element using the ferroelectric thin film coated substrate.
Since ferroelectrics has a lot of advantages, such as spontaneous polarization, high dielectric constant, an electro-optic effect, a piezoelectric effect and a pyroelectric effect, it is applied to the development of various devices, such as a capacitor, an oscillator, a light modulator and an infrared sensor. Conventionally, in these applications, monocrystal, made of triglycine sulfate (TGS), LiNbO3 and LiTaO3 which are materials of the ferroelectrics, or a ceramic, which is made of BaTiO3, PbTiO3, Pb (Zr1xe2x88x92Tix)O3 (PZT), PLZT, etc., was cut and was ground so as to have a thickness of approximately 50 xcexcm. However, it is difficult and costly to produce large-sized monocrystal, and its processing is difficult due to cleavage. Moreover, since ceramics are generally fragile and it is difficult to process the ceramics so that it has a thickness of less than 50 xcexcm due to a crack, etc. at a processing step, a lot of effort is required and its production cost becomes higher.
Meanwhile, as a technique for forming a thin film is developed, an application field for such a ferroelectric thin film is spreading at present. As an example of the applications, when high dielectric constant properties are applied to a capacitor for various semiconductor devices, such as DRAM, high integration of a element is realized by decreasing the capacitor size and reliability is improved. In particular, a high density ferroelectric nonvolatile memory (FRAM) which is operated at high speed has been developed by combining a ferroelectric thin film and a semiconductor memory element, such as DRAM, recently. The ferroelectric nonvolatile memory does not require backup battery because of utilization of the ferroelectric properties (hysteresis effect) of the ferroelectrics. The development of these devices require materials which have properties, such as large remanent spontaneous polarization (Pr), a small coercive electric field (Ec), a low leakage current and excellent endurance to repetition of polarization reversal. Moreover, in order to lower an operating voltage and to suitably perform semiconductor fine processing, it is desirable that the above properties are realized by a thin film with a thickness of less than 200 nm.
In order to achieve the application of the ferroelectric thin film to the FRAM, etc., an oxide ferroelectric thin film having a perovskite structure, such as PbTiO3, PZT, PLZT, etc. is tried to be formed by thin film forming methods, such as sputtering method, vacuum evaporation method, sol-gel method and MOCVD (Metal-Organic Chemical Vapor Deposition) method.
In the above ferroelectric materials, Pb (Zr1xe2x88x92xTix)O3 (PZT) is now being studied most intensively, and a thin film with excellent ferroelectric properties is obtained by the sputtering method and the sol-gel method. For example, a thin film, whose remanent spontaneous polarization Pr takes a large value in the range of 10 xcexcC/cm2 to 26 xcexcC/cm2, is obtained. However, although the ferroelectric properties of PZT greatly depend upon composition x, PZT contain Pb whose vapor pressure is high, so decrease in a film thickness arises a problem that the leakage current and fatigue in the endurance to polarization reversal are caused because film component is liable to change at the time of film formation and of heat treatment, a pinhole is produced, a low dielectric constant layer is obtained due to reacting between a ground electrode Pt and Pb, etc. For this reason, it is desired that another materials whose ferroelectric properties and the endurance to polarization reversal are excellent are developed. Moreover, in the case of the application to an integrated device, a fine grain of a thin film, which corresponds to fine processing, is required.
Bi layered oxide materials such as SrBi2Ta2O9 have attracted interest as the materials with fatigue free property. A thin film of SrBi2Ta2O9 is produced by a MOD method. The MOD method is a method for forming a film including the following processes. Namely, like the sol-gel method, metalorganic raw materials are mixed so that fixed film composition is obtained, and a raw material solution for application whose concentration and viscosity are adjusted is produced. A substrate is spin-coated with the produced raw material solution and the substrate is dried. Then, in order to remove the organic element and solvent, the substrate is heated to form the amorphous film. These processes are repeated until the fixed film thickness is obtained, and finally, the substrate is crystallized by sintering. Therefore, the film thickness is controlled by adjusting the thickness of a once-applied film (see Extended Abstracts (The 55th Autumn Meeting, 1194): The Japan Society of Applied Physics, 20 p-M-19).
The most serious problem of SrBi2Ta2O9 thin film formation is that since the sintering temperature is very high, namely, 750xc2x0 C.-800xc2x0 C., a long sintering time, namely, longer than 1 hour, is required. In such a manufacturing process, when the processes, such as film formation and heat treatment are performed for a long time at the temperature of not lower than 650xc2x0 C., a mutual diffusion reaction between a platinum metal electrode as a substrate and the ferroelectrics and reactions between silicon or oxidized silicon under the ground electrode and the electrode or the ferroelectrics are taken place. Moreover, the film composition is changed due to volatilization of a composition element from the ferroelectric thin film, and thus the application to the actual device producing process becomes difficult. Moreover, since only a film, having surface morphology of a large grain size of approximately 0.3 am, is obtained at present, the film cannot be applied to the submicron fine processing which is required for the development of highly integrated devices. Moreover, in the case of the coated film, since a coating method is disadvantageous of a step coverage, there arises a problem of disconnection of a wiring, etc. Therefore, as to SrBi2Ta2O9, its ferroelectric properties and its fatigue free property are excellent, but it still has a serious problem for application to devices.
In addition, in order to realize high integration of the ferroelectric nonvolatile memory at present, it is proposed to use a polycrystal silicon plug for wiring between a MOS transistor and a ferroelectric capacitor, but in the case where a ferroelectric thin film is produced by a long-time and high temperature process used for SrBi2Ta2O9, there arises a problem that its properties deteriorate. due to mutual diffusion between the polycrystal silicon used for wiring and the ferroelectric thin film. In order to solve such a problem, a structure where various diffusion barrier layers are inserted is examined, but even in such a structure, the permitted limit of a forming temperature of the ferroelectric thin film is up to 650xc2x0 C., and on another short-time heat treatment process, the permitted limit is up to approximately 700xc2x0 C. However, at present in the ferroelectric thin film made of SrBi2Ta2O9 or the like, in general, as the film forming temperature is higher, the ferroelectric properties as well as crystallinity are more improved. Therefore, when the film forming temperature is lowered, the crystallinity and the ferroelectric properties are deteriorated, so it is difficult to achieve both the improvement in the ferroelectric properties and the low film forming temperature in the ferroelectric thin film.
On the other hand, an example of oxide ferroelectrics excluding Pb, which exerts a bad influence upon the leakage current and the resistance to polarization reversal, is Bi4Ti3O12 having a layered perovskite structure. Such Bi4Ti3O12 is ferroelectrics having the layered perovskite structure whose anisotropy is strong (orthorhombic system/lattice constants: a=5.411 xc3x85, b=5.448 xc3x85, c=32.83 xc3x85). As to ferroelectricity of its monocrystal, in the a-axis, the remanent spontaneous polarization Pr is 50 xcexcC/cm2 and the coercive electric field Ec is 50 kV/cm, namely, the ferroelectrics has the largest spontaneous polarization in the above Bi oxide ferroelectrics so Bi4Ti3O12 shows excellent properties. Therefore, in order to apply the large spontaneous polarization of Bi4Ti3O12 to the ferroelectric nonvolatile memory, etc., it is desirable that the Bi4Ti3O2 has a lot of a-axial composition of the crystal in the direction which is perpendicular to the substrate.
Thinning of Bi4Ti3O12 thin film has been tried by the MOCVD method and the sol-gel method, but these attempts mostly brought c-axis-oriented films whose spontaneous polarization is smaller than a-axis-oriented films. Moreover, in the conventional sol-gel method, the heat treatment of not lower than 650xc2x0 C. is required for obtaining the excellent ferroelectric properties, and since its surface morphology is composed of crystal grains of approximately 0.5 xcexcm, it is difficult to apply the thin film to the highly integrated devices which requires fine processing.
Meanwhile, a c-axis-oriented Bi4Ti3O12 thin film is formed by the MOCVD method on a Pt/SiO2/Si substrate and a Pt substrate at a substrate temperature of not lower than 600xc2x0 C., but these substrates cannot be directly applied to an actual device structure. In other words, like the Pt/Ti/SiO2/Si substrate, an adhesive layer, such as a Ti film, is required for obtaining adhesive strength between a Pt electrode layer and SiO2 below it. However, in the case where the Bi4Ti3O12 thin film is formed by the MOCVD method on the Pt electrode substrate to which such an adhesive layer is provided, it is reported that its film surface morphology is composed of coarse crystal grains and that pyrochlore phase (Bi2Ti2O7) is liable to occur (see Jpn. J. Appl. Phys., 32, 1993, pp.4086, and J. Ceramic Soc. Japan, 102, 1994, pp.512). In the case where the film surface morphology is composed of coarse crystal grains, the film cannot be applied to the highly integrated devices which require fine processing, and furthermore, a thin thickness causes a pinhole, thereby generating a leakage current. Therefore, in such a conventional technique, it is difficult to realize the ferroelectric thin film which has excellent ferroelectric properties in the case of a thin film thickness of not more than 200 nm.
As mentioned above, in order to apply a ferroelectric thin film to highly integrated devices, the above prior art cannot provide a ferroelectric thin film, which sufficiently fulfills various conditions, such as denseness and evenness on the thin film surface required for fine processing and a low leakage current, large remanent spontaneous polarization, and a film forming process at a low temperature.
It is an object of the present invention to provide a ferroelectric thin film coated substrate, where a ferroelectric thin film with a dense and even surface, an excellent low leakage current properties and sufficiently large remanent spontaneous polarization, can be produced at a lower temperature, a method of producing the ferroelectric thin film coated substrate and a capacitor structure element using the ferroelectric thin film coated substrate.
In order to achieve the above object, a ferroelectric thin film coated substrate of the present invention is produced by the following characteristic steps of:
forming a metal oxide buffer layer on a substrate;
forming a first crystalline ferroelectric thin film with a first film thickness on the metal oxide buffer layer at a first substrate temperature; and
forming a second ferroelectric thin film with a second film thickness thicker than the first film thickness on the first ferroelectric thin film at a second substrate temperature lower than the first substrate temperature.
In accordance with the above arrangement, since the first ferroelectric thin film is positioned through the metal oxide buffer layer, even when the first ferroelectric thin film has a thin film thickness, a thin film having satisfactory crystallinity can be formed. Therefore, in the case where the first ferroelectric thin film is used as a background layer, even if the second ferroelectric thin film with a thicker second film thickness at the film forming temperature (substrate temperature) which is lower than the film forming temperature (substrate temperature) of the first ferroelectric thin film, sufficient ferroelectricity can be secured due to inheritance of excellent crystallinity of the first ferroelectric thin film. Moreover, when the second ferroelectric thin film is formed at a lower temperature, the crystal grains composing the thin film can be prevented from becoming rough and large, thereby making it possible to obtain the dense ferroelectric thin film whose surface is even.
In other words, when the ferroelectric thin film coated substrate of the present invention is arranged so that the second ferroelectric thin film with an enough film thickness for showing ferroelectricity is positioned on the substrate through the metal oxide buffer layer and the first ferroelectric thin film with a thin film thickness, the ferroelectric thin film which has sufficient ferroelectricity and has excellent evenness and denseness can be obtained.
In addition, in the ferroelectric thin film coated substrate of the present invention, since the ferroelectric thin film has excellent evenness and denseness, fine processing becomes possible, and the substrate is applicable to various highly integrated devices. Moreover, if the substrate is applied to various devices including the capacitor structure element of the present invention, production of a pin hole is prevented, thereby making it possible to improve the leakage current properties.
It is desirable that the ferroelectric thin film coated substrate of the present invention is produced such that after the first ferroelectric thin film is formed on the metal oxide buffer layer positioned on the substrate by heating the substrate by means of the MOCVD method, the second ferroelectric thin film is formed by the MOCVD method at a substrate temperature which is lower than that in forming the first ferroelectric thin film.
In addition, it is desirable that the substrate temperature in forming the first ferroelectric thin film (first substrate temperature) is in the range of 450xc2x0 C. to 650xc2x0 C., and the substrate temperature in forming the second ferroelectric thin film (second substrate temperature) is in the range of 400xc2x0 C. to 500xc2x0 C. (however, the range of 450xc2x0 C. to 500xc2x0 C. is limited to the case where the second substrate temperature is lower than the first substrate temperature).
In the producing method of the present invention, the substrate temperature in forming the first ferroelectric thin film is slightly higher, but it is enough lower compared with the conventional method, and the process for forming the first ferroelectric thin film requires a short time because its thickness may thin. Therefore, there is little effect of the substrate temperature.
Therefore, in accordance with the method of producing the ferroelectric thin film coated substrate of the present invention, since the ferroelectric thin film is formed at a very low temperature during almost the whole process, the substrate is applicable to a highly integrated device provided with a plurality of elements without damaging another elements. Furthermore, a degree of freedom in design can be remarkably improved.